Showing a notification doesn’t “evade” battery optimization - as battery optimization is completely independent of Android’s memory management.
Showing a notification allows Android’s memory management to better assess what apps to kill using a weights system, based on whether they are in the foreground or background, if any of the app’s overlays/notifications are visible to the user, and exactly how visible they are.
Battery optimization will kill any non-system app based on how frequently it is used, dependent on its overall background CPU usage, regardless of whether the app is showing a notification or not.